According to a new report out from George Washington University, 45% of births in the U.S. in 2010 were paid for by Medicaid, up slightly from 43% in 2008. That amounts to 1.8 million of the estimated 4 million births in the U.S. in 2010. The report has been published in this month’s issue of the journal Women’s Health Issues.

The study‘s authors say the estimate will help other researchers as they observe the coming change in health care. Many provisions of the Patient Protection and Affordable Care Act, colloquially known as Obamacare, are scheduled to take effect at the beginning of 2014. In concert with the legislation, some states will be expanding their medicaid coverage, particularly to low-income women. These expansions could also lead to an expansion of maternity care.

“As states expand coverage, low-income women of childbearing age will be able to obtain more continuous coverage before and between pregnancies,” said Anne Markus, lead author of the study and a professor of health policy at George Washington University. “Now, for the first time, researchers will have a comprehensive baseline that will help them determine how increased access to services might change pregnancies and ultimately birth outcomes.”

Markus and her colleagues collected data on births paid for by Medicaid between 2008 and 2010. In addition to the overall Medicaid birth numbers, the study found that percentages varied wildly for different states. States in the northeast and northwest portions of the U.S. had the lowest percentages of births paid for by Medicaid (Massachusetts had only 30%), while southern states such as Mississippi, Arkansas, and Louisiana (which had a whopping 70%) have higher percentages.

With the new study, researchers hope is to provide enough data that health officials can use to determine how Medicaid policy effects maternal and child health.

“About half a million babies are born prematurely in the United States every year,” said Dr. Jennifer Howse, president of the March of Dimes, which participated in the research. “Some of these preterm births could be prevented with the appropriate care provided at the right time. Babies born premature are at risk for lifelong health problems and often require care in a hospital’s neonatal intensive care unit. This study gives us a critical baseline to help chart the progress of health reform as it affects maternal and child health.”